1. Field of the Invention
The present invention relates to a method of breakdown formation which is an initial step for forming an electro-unite tube. Particularly, the invention relates to a method of breakdown formation for forming a metal band plate by using breakdown pass rolls and forming both side portions thereof into an arcuate shape.
2. Prior Art
FIG. 6 illustrates a step of forming an electro-unite tube, in which a metal band plate 1 is formed into an arcuate shape at its both side portions through a series of breakdown pass rolls 2, and is, then, gradually formed into a circular shape in cross section through fin pass rolls 3. Finally, both edges are abutted together through squeeze rolls (not shown) and are welded together by electric resistance heating.
FIG. 7 illustrates a conventional breakdown pass roll 2, wherein the circumferential surface of an upper roll 2a includes flat central surfaces and convex curved surfaces at both the right and left side portions thereof. The circumferential surface of a lower roll 2b is symmetrical to the circumferential surface of the upper roll 2a and includes flat central portions, and concave curved surfaces at both the right and left side portions. A metal band plate 1 is held between the upper roll 2a and the lower roll 2b, and both side portions thereof are formed in an arcuate shape.
FIG. 8 illustrates another conventional breakdown pass roll 2, in which the upper right and left rolls 2c, 2c are provided being spaced apart from each other, and the circumferential surface of the lower roll 2d includes concave portions formed in both the right and left side portions thereof so as to meet the curved surfaces of the upper rolls 2c, and a slightly convex curved surface at the central portion thereof. The metal band plate 1 is held between the upper rolls 2c and the lower roll 2d, and is formed into an arcuate shape at both side portions thereof.
In the conventional breakdown pass roll, the concave portion is formed in the lower roll to meet the convex curved surface of the upper roll, and the metal band plate is held between the upper roll and the lower roll being intimately adhered thereto. With the combination of the same upper roll and the same lower roll, therefore, both side portions of a metal band plate are formed into an arcuate shape maintaining the same curvature. When it is attempted to mold an electro-unite tube having a different plate thickness or a different diameter, therefore, there must be employed a combination of another upper roll and another lower roll. Besides, the metal band plate is molded at a forming portion in a state of being intimately adhered to the upper roll and the lower roll at all times, resulting in the occurrence of hardening due to working and adversely affecting the quality of the electro-unite tube.
In forming the electro-unite tubes having different plate thicknesses and diameters, therefore, there arises a technical problem that must be solved so that the breakdown formation can be accomplished without the need of changing the combination of the upper roll and the lower roll each time. The object of the present invention is to solve this problem.